1. Field of the Invention
This invention relates in general to well logging radioactive detecting equipment, and in particular to a Dewar flask for containing the detector, the Dewar flask having a window of low gamma ray attenuation.
2. Description of the Prior Art
One type of means of logging a well is by the use of a radioactive source. An instrument is lowered into the well containing a radioactive source and a gamma ray detector. The source emits radioactive energy into the formation of the earth. Gamma rays occurring as a result of the emission of the radioactive energy will be detected by the detector. The rate of gamma rays detected can be used to determine characteristics of the formation, such as the density.
One type of detector operates better at temperatures lower than those encountered in a well. A well bore may be at several hundred degrees, while the detector may operate better at normal ambient conditions at the surface. A Dewar flask is used to contain the detector to maintain it as near as possible at ambient surface temperatures. The Dewar flask has inner and outer containers separated by a space. The space is evacuated for insulation.
Incoming gamma rays must pass through both the inner and outer housings before reaching the detector. For accurate reading, the inner and outer housings should have as low of attenuation as possible. They should be as transparent to gamma rays as possible so as to not capture any low energy gamma rays that otherwise would reach the detector.
In one prior art type, a window will be located adjacent the detector. This window is made up of a material such as beryllium that has a low Z (atomic number) material. A low Z material will transmit low energy gamma rays better than a high Z material. The remaining portions of the outer housing of the flask are made up of a high Z material. The two different materials required sealing at the junctions. The windows of the prior art type are expensive and difficult to fabricate. The outer housing of the flask has to be fairly sturdy to withstand collapsing due to the vacuum contained within the outer housing.